Air Purification Units (APUs) are used to extract contaminants and undesirable compounds (henceforth “contaminants”) from an air stream such that a “purified” air stream is produced for a process. These typically operate using an adsorption process whereby the contaminants, which may be gases, water molecules, hydrocarbon particles or any other undesired species, are adsorbed onto the surface of an adsorbent material. The adsorbent material is carefully selected to preferentially adsorb the contaminants that the process designer wishes to remove. There are two types of adsorption: physical and chemical.
APUs are well known in the art of air liquefaction. They are used to produce a clean, dry stream of air to be liquefied—notably avoiding fouling of the process as contaminants freeze, and ensuring a pure liquid air product. Typically, the APU of an air liquefier is designed to remove carbon dioxide, moisture and hydrocarbons.
An APU typically consists of a vessel containing a particulate bed of adsorbent material through which the process stream flows. Since the adsorbent capacity of an adsorbent material is finite, APUs operate in two principal phases: adsorption and regeneration (otherwise known as desorption). Adsorption is an exothermic process, releasing heat. Desorption requires the addition of heat.
Two of the main process parameters affecting adsorption are pressure and temperature, which may be manipulated to alter the equilibrium between the fluid and the adsorbent. In a physical adsorption process, adsorption increases at higher pressure and decreases at higher temperature. In a chemical adsorption process, the relationship with temperature is often more complex. For simplicity, the following description concentrates on physical adsorption but the principles of the present invention may equally be applied to chemical adsorption processes. Equally, the following description concentrates on the use of APUs for air liquefaction; however, a person skilled in the art will recognise that the principles of the present invention apply to any similar application.
During the adsorption process cycle, pressure and temperature are controlled such that the adsorbent material adsorbs during the adsorption phase and desorbs during the regeneration phase. In what is known in the art as a Pressure Swing Adsorption process, pressure is controlled so that it is high during the adsorption phase and low during the regeneration phase. In what is known in the art as a Temperature Swing Adsorption process, temperature is controlled so that it is low during adsorption and high during regeneration.
In a combined cycle, during the adsorption phase, the pressure of the process air stream is high and the temperature is low so that contaminants are adsorbed onto the surface of the adsorbent material. During the regeneration phase, a lower-pressure, higher temperature regeneration gas stream (which may be air or otherwise) is flowed through the bed. As a result, the equilibrium between the gas stream and the adsorbent material is changed such that contaminants are desorbed from the adsorbent material into the gas stream. The regeneration gas stream is then typically exhausted to atmosphere in order to remove the contaminants from the system. The regeneration phase is usually followed by a cooling phase where the adsorbent bed is cooled, using a cooler stream of gas, to a lower temperature before recommencing the adsorption phase. The lower the temperature of the bed, the more efficient the adsorption.
Since the adsorption and regeneration phases are both necessary, in order to achieve a continuous flow of purified air to the process, APUs in air separation plants predominantly consist of two vessels, of which one adsorbs while the other is regenerated and then cooled. Once the effective capacity of the adsorbing vessel is reached (saturation), the flow paths are swapped using a system of valves so that the regenerated vessel becomes the adsorbing vessel and the “full” vessel begins regeneration.
The phenomenon of adsorption exhibits a number of characteristics which the designer must account for when designing an adsorption system.
As an adsorbent bed adsorbs contaminants from the process stream, a concentration front moves through the vessel. Upstream of this front, the adsorbent material is saturated with contaminant and downstream of this front the adsorbent material is “fresh”. In reality, this front is not a discontinuity but a concentration gradient between saturated adsorbent upstream and fresh adsorbent downstream. The zone occupied by this gradient is often referred to as the “mass transfer zone” as this is where mass is transferred from the fluid to the adsorbent during adsorption and from the adsorbent to the fluid during regeneration.
The mass transfer zone will traverse the length of the adsorbent bed at a velocity often referred to as the wave velocity. This determines the time required for the mass transfer zone to traverse the adsorbent bed, and therefore the amount of time to complete the adsorption phase or the regeneration phase.
The length and velocity of the mass transfer zone depends on a number of process parameters, including, for example, the adsorbent used, the size of the adsorbent particles and the velocity of the flow. The shape and velocity of the mass transfer zone generally differ between the adsorption and regeneration phases. Moreover, the shape and velocity of the mass transfer zone may change with time during the cycle.
During the adsorption process, the flow through the adsorption vessel must be stopped before an unacceptable concentration of contaminants arrives at the outflow, when the leading edge of the mass transfer zone arrives at the end of the vessel. In the region of the mass transfer zone, the adsorbent is not fully saturated and the full capacity of the bed has not been used. While the process designer may be able to control the shape and speed of the mass transfer zone to a certain extent, the mass transfer zone will inevitably occupy a portion of the length of the vessel. The shorter the vessel, the larger the relative portion occupied by the mass transfer zone. It is therefore desirable to design the adsorption vessel with sufficient length such that the area occupied by the mass transfer zone is proportionally small and a minimum of adsorbent remains unsaturated at the end of the adsorption phase.
This problem is not generally encountered during the regeneration phase since the regeneration stream is exhausted to atmosphere and one is not concerned with the concentration of contaminants.
A technique for improving the utilisation of the adsorbent bed, which is known in the art of adsorption processes (as described, for example, in Wankat, Phillip C. (1986). Large-Scale Adsorption and Chromatography, Volumes 1-2) but is not disclosed for use in any particular application other than waste water treatment, consists of two columns in series whereby the mass transfer zone may be entirely “pushed” out of one column and into the other so as to fully utilise the bed from the first column. An exemplary implementation of this system in the art comprises three identical columns, of which one regenerates while the remaining two adsorb. The two adsorbing columns are arranged in series in the flow such that the mass transfer zone may overrun from the first column into the second, allowing the first column to be fully saturated. During this time the third column is regenerated. Once the first column is saturated, the third regenerated column is connected in series with the second to capture the mass transfer zone as it exits the second column. Meanwhile the first column is regenerated. By continuing these steps in a cyclical fashion, continuous adsorption may be performed while utilising the full capacity of the adsorbent beds; the mass transfer zone is effectively consistently “pushed” into the newly regenerated vessel. FIG. 1 illustrates the two phases described above.
Another key consideration for the designer of a system is pressure drop—a higher pressure drop equates to more wasted energy. The flow of air through a particulate bed of adsorbent experiences a pressure drop that is primarily a function of the size of the adsorbent particles, the length of the bed and the superficial flow velocity.
Larger particles result in a lower pressure drop but less effective adsorption. Pressure drop may also be reduced by limiting the length of bed or reducing the velocity of the flow through it.
While the velocity of the flow is important for pressure drop, it is most important to maintain low velocity in order to remain below the fluidisation velocity of the adsorbent particles. The fluidisation velocity is the velocity at which the adsorbent particles begin to move due to the forces exerted on them by the moving fluid. This can cause layers of different adsorbent types to mix and may result in contamination of the wider process with adsorbent leaving the APU.
In order to maintain low velocity, it is well understood in the art that for a vessel of length L and diameter d containing a given quantity of adsorbent, velocity may be reduced by selecting a smaller length to diameter ratio (L/d). This has the effect of increasing the cross-sectional flow area, resulting in lower flow velocity. Furthermore, the shorter vessel length will contribute to a lower pressure drop.
However, cost considerations lead the designer to limit the diameter of the vessel. Furthermore, if the vessel diameter is too great, the flow may not be well distributed within it and dead zones may exist around the circumference at the extremities where little or no mass transfer occurs between the fluid and the adsorbent.
There is also a motivation to maintain sufficient length in the vessel so that the mass transfer zone does not occupy a large proportion of the length of the adsorbent bed.
While it is desirable for the above reasons to limit flow velocity, it is preferable to keep the flow velocity high enough so that axial dispersion is not a dominant mass transfer mechanism, as axial dispersion tends to reduce the efficiency of the adsorption process by elongating the mass transfer zone.
It is therefore known in the art that there is a trade-off in the design of the APU with regard to the different requirements of the process and the cost of building the system.
A further consideration in the design of an APU is the source of the regeneration stream. In state-of-the-art air liquefiers, the regeneration gas stream is primarily sourced from the clean input air stream, a portion of which is diverted, expanded to a lower pressure, heated and used to regenerate the regenerating vessel.
FIG. 2 shows a simplified example of a state-of-the-art regeneration scheme, wherein a stream of feed air from ambient is drawn into compressor 100 where it is compressed. The air stream flows through adsorption vessel 111 where contaminants are removed by adsorption. Now consisting of clean, dry air, the air stream is split into a process air stream and a regeneration air stream. The process air stream is supplied to a cold box 120 which forms part of an air liquefier. The regeneration air stream is let down to lower pressure in valve 201 and flows through heating device 101 where heat is added to raise the temperature to the required regeneration temperature. The required regeneration temperature at the outflow of heating device 101 depends notably on the adsorbent material(s) used and the desired concentration to be achieved. The warmed regeneration air stream is then flowed through the regeneration vessel 112, where contaminants are desorbed into the regeneration air stream and evacuated with it to atmosphere. Once regeneration vessel 112 is regenerated, heating device 101 is turned off and the now cooler regeneration stream is used to cool the adsorbent in regeneration vessel 112. The regeneration and cooling nominally last the same time as the adsorption process.
However, it will be appreciated that the above method requires the feed air compressor 100 to be over-sized in order to provide the extra flow rate required for regeneration, which is ultimately wasted to atmosphere and not liquefied.
Alternatively, where another gas stream is available, this may be used to regenerate the APU. EP2510294 describes an air separation plant wherein air is liquefied in an air liquefier and separated into its component parts in a cryogenic distillation column. A portion of the resulting pure nitrogen is used to regenerate the APU before being vented to atmosphere while the oxygen component is a final product stream. This method is used only where there is not sufficient demand for the nitrogen product.
In state of the art APUs, the flow of the regeneration stream is continuously available as long as there is a process stream to be purified. In such cases, the APUs, and the systems in which they are used, operate well. However, several problems have been met when using APUs in cryogenic energy storage systems such as liquid air energy storage (LAES) systems. Such systems are known to provide an effective means of storing energy on a large scale to balance consumer demand for electricity with electricity generating capacity, and to smooth out levels of intermittent supply from, for example, renewable energy sources.
WO2007/096656 and WO2013/034908 disclose cryogenic energy storage systems which exploit the temperature and phase differential between low temperature liquid air and ambient air, or waste heat, to store energy at periods of low demand and/or excess production, allowing this stored energy to be released later to generate electricity during periods of high demand and/or constrained output. The systems comprise a means for liquefying air during periods of low electricity demand, a means for storing the liquid air produced, and a series of expansion turbines (or a series of stages of an expansion turbine) for expanding the liquid air. The expansion turbine(s) are connected to a generator to generate electricity when required to meet shortfalls between supply and demand.
An advantage of LAES over other energy storage technologies is that the liquefaction of air may be decoupled from power recovery such that the rates of charge and discharge, and the quantity of energy stored as liquid air are all independent (i.e. the respective stages of operation take place separately; that is singly and usually consecutively, rather than concomitantly). The differing charge and discharge rates are referred to as asymmetric operation and allow, for example, slow charging overnight and rapid discharge of the stored energy over only a few hours of peak electricity demand during in the day. This is known as ‘asymmetric operation’ and charge times may be several times longer than discharge times.
In a conventional air liquefaction plant, a stream of clean, dry air is required for liquefaction, and an APU must be employed.
In a state-of-the-art LAES system, during the charging phase when air is being liquefied, a regeneration scheme typical of traditional air liquefaction plants is used, as shown in FIG. 2. In such a system, a portion of the process air is diverted via a heating device to the regeneration vessel and then vented to ambient to remove the contaminants from the system. Thus, it is necessary to sacrifice a portion of the input air as no waste gas streams are generally available for regeneration, and this is undesirable.
One of the key parameters of a commercially viable energy storage system is the round-trip efficiency, which represents the portion of the energy input to the system that is recovered following storage. It is desirable to minimise the energy required to produce liquid air in the liquefier and maximise the energy extracted from the air in the power recovery unit.
In order to optimise the round-trip efficiency of LAES systems, there is a need to reduce the power required for the regeneration of the APU, and thus avoid sacrificing a portion of the input air.
Furthermore, there is a need to better adapt an APU to operate efficiently within the constraints imposed by the asymmetric operation of LAES (i.e. without the two-phase continuous adsorption cycles described above, wherein the regeneration stream is available for the same duration as the adsorption phase).